Cheers! Yeah, the genre isn't for everyone, kudos for giving it a go.
Saranhil is inexplicably attracted to the MC for the simple reason that this is an isekai parody. And well, frankly I've seen far weirder reasons for people to be attracted to one another in real life, so I don't see much of a point in sweating the details in fiction.
Ruru's motivations are so far only hinted at.